Assisted-Global Positioning System (A-GPS) has emerged as a strong candidate for providing accurate location determination for mobile terminals operating in wireless telecommunications networks. In addition to the hardware typically associated with mobile handsets in a radio telecommunications network an A-GPS equipped mobile terminal has a satellite positioning system receiver (or partial satellite positioning system receiver) configured to determine the location of the handset. The satellite positioning system receiver relies on the wireless communications network to communicate so called “assistance data” to the mobile station to improve, inter alia, positioning sensitivity, signal acquisition time, accuracy, and battery consumption without requiring additional hardware.
By providing assistance data to an A-GPS receiver weaker signals can be detected, thereby making it possible in some instances to obtain a location fix for the A-GPS receiver where it would not have been possible with a traditional GPS receiver.
Another advantages of A-GPS over conventional satellite positioning system receivers is the reduced time to first fix realised by A-GPS receivers. However, particularly for emergency calls (e.g. E911 and E112) it is desirable that this time be further reduced.
One of the key ways to improve the sensitivity of an A-GPS handset, and potentially decrease the time to first fix, is by providing signal acquisition assistance data to the handset. In use a satellite positioning system receiver detects a satellite signal by correlating the received signal with a locally-generated copy of the signal. Each of the signals comprises a repeating 1023 bit (chip) pseudorandom code and the offset between their respective phases represents a measure of the range between the satellite and the A-GPS handset.
Without assistance data the satellite positioning system receiver must search the entire space of possible satellites, frequency offsets and code delays to attempt to find a match between a received signal and a locally generated signal and to thereby determine the range of the receiver from the satellite.
FIGS. 1A and 1B of the accompanying drawings represent the search space for a satellite positioning system receiver. FIG. 1A represents a full three-dimensional search space 100 for a satellite positioning system receiver comprising a group of two-dimensional search spaces 102a . . . 102f. Each of the two-dimensional search spaces corresponds to a particular satellite in the satellite positioning system constellation
One of the key pieces of assistance data provided to an A-GPS mobile terminal is an indication of those satellites in the satellite positioning system constellation from which the mobile terminal should be able to acquire a signal. This assistance data limits the three dimensional search space to a reduced set of search spaces, each of which should yield a code phase match for a received satellite signal.
FIG. 1B shows the two-dimensional search space (e.g. 102a of FIG. 1A) for a particular satellite in the satellite positioning system constellation from which the A-GPS receiver should be able to acquire a signal. The search space 102a for the satellite is divided into a plurality of “bins” 104 which represent discrete frequency and code delay increments through which the A-GPS receiver must search to obtain a code phase correlation as detailed above.
The code phase search space for a given signal is determined by the possible range of signal travel times between the satellite that transmitted the signal and the satellite positioning system receiver. For example, satellite positioning system satellites orbit the earth at approximately 20,180 km above its surface. The travel time of a signal transmitted from a satellite at zenith is 76 milliseconds, and for a satellite on the horizon is 86 milliseconds. Accordingly, there is a 19 millisecond window in which the signal from any given satellite can be received depending on its position in the sky relative to the A-GPS receiver. As the chipping rate of a global positioning system signal is 1023 kilohertz this corresponds to a maximum code phase search window in the order of 19,000 bits.
By estimating the position of the A-GPS terminal and assigning an associated accuracy the possible range between the A-GPS terminal and the satellite is reduced. This results in a reduced code phase search window which can be communicated to the A-GPS terminal as assistance data in an attempt to reduce the time spent by the terminal looking for a code phase match for a received signal. As will be appreciated by those skilled in the art a reduction in the range of possible Doppler frequencies may also be made and communicated to the A-GPS terminal as assistance data. Thus rather than the A-GPS terminal having to search for a signal over the whole two dimensional search space 102a shown in FIG. 1B the search region can be reduced to the code phase search region indicated by reference numeral 106 in FIG. 1B.
The present inventor has recognised that a smaller search window can be provided to an A-GPS handset by reducing the code phase range over which the search must be performed.